Elevational Gradients in β-Diversity Reflect Variation in the Strength of Local Community Assembly Mechanisms across Spatial Scales

Abstract

Despite long-standing interest in elevational-diversity gradients, little is known about the processes that cause changes in the compositional variation of communities (β-diversity) across elevations. Recent studies have suggested that β-diversity gradients are driven by variation in species pools, rather than by variation in the strength of local community assembly mechanisms such as dispersal limitation, environmental filtering, or local biotic interactions. However, tests of this hypothesis have been limited to very small spatial scales that limit inferences about how the relative importance of assembly mechanisms may change across spatial scales. Here, we test the hypothesis that scale-dependent community assembly mechanisms shape biogeographic β-diversity gradients using one of the most well-characterized elevational gradients of tropical plant diversity. Using an extensive dataset on woody plant distributions along a 4,000-m elevational gradient in the Bolivian Andes, we compared observed patterns of β-diversity to null-model expectations. β-deviations (standardized differences from null values) were used to measure the relative effects of local community assembly mechanisms after removing sampling effects caused by variation in species pools. To test for scale-dependency, we compared elevational gradients at two contrasting spatial scales that differed in the size of local assemblages and regions by at least an order of magnitude. Elevational gradients in β-diversity persisted after accounting for regional variation in species pools. Moreover, the elevational gradient in β-deviations changed with spatial scale. At small scales, local assembly mechanisms were detectable, but variation in species pools accounted for most of the elevational gradient in β-diversity. At large spatial scales, in contrast, local assembly mechanisms were a dominant force driving changes in β-diversity. In contrast to the hypothesis that variation in species pools alone drives β-diversity gradients, we show that local community assembly mechanisms contribute strongly to systematic changes in β-diversity across elevations. We conclude that scale-dependent variation in community assembly mechanisms underlies these iconic gradients in global biodiversity.

Dispersal enhances beta diversity in nectar microbes

“…(), and Tello et al . (). Specifically, within each dispersal treatment, null communities were generated by subsampling with fixed regional species abundance distributions, following Tello et al .…”

“…Specifically, within each dispersal treatment, null communities were generated by subsampling with fixed regional species abundance distributions, following Tello et al . (). For empirical and null communities separately, the distance to the treatment centroid was then extracted, with the number of points equivalent to that in the corresponding treatment group.…”

“…Note that our beta deviation, which is calculated for each replicate community, is different from Tello et al . 's (), which computes a single measure of beta deviation per treatment. We used the modified, per‐replicate calculation because it enables statistical tests of treatment effects on beta deviation.…”

Towards connecting biodiversity and geodiversity across scales with satellite remote sensing

“…We expect that of all the forms of biodiversity, beta diversity will be linked most strongly with heterogeneity in geodiversity, because variation in geodiversity can lead to concomitant shifts in abiotic resource availability that alter habitat types and drive species turnover (Ricklefs, ). Biodiversity–geodiversity relationships are likely to be scale dependent owing to varying influences of local community assembly processes, such as dispersal limitation, biotic interactions and environmental filtering (e.g., Tello et al, ).

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Small montane cloud forest fragments are important for conserving tree diversity in the Ecuadorian Andes

“…, Tello et al . , Wilson and Rhemtulla. However, to account for potential undersampling in these very diverse environments, we used rarefaction to derive estimates of species richness, as well as richness estimators that consider rare species.…”